1. Field of the Invention
The present invention relates to electronic circuits and systems sensitive to electrostatic discharge, and more particularly, to a distributed electrostatic discharge protection network for these electronic circuits and systems operating at radio frequencies.
2. Description of the Related Technology
Electrostatic charge was discovered by the early Greeks and was a novelty until electricity became better understood and more widely used. An electrostatic charge potential may become so large that the insulation medium between the positive and negative charges breaks down. This break down results in what is called “electrostatic discharge” or “ESD.” Examples of minor and major ESD events are an electric spark from a metal door knob to one's finger after shuffling across a carpeted floor, and a lightning bolt between the earth and the clouds, respectively. Electrical and electronic devices and systems are sensitive to ESD because insulation breakdowns, caused by ESD, may seriously degrade the electrical performance characteristics of the device or system. Electrical power systems are protected from ESD (lightning) by protective devices having voltage break down characteristics that short out and dissipate the ESD event before it can damage the electrical system.
Electronic circuits and systems have become more susceptible to ESD damage as the circuit elements thereof have become smaller and the insulation thinner therebetween. During the era of vacuum tube technology, the vacuum tube's operating voltages were hundreds or even thousands of volts, and the spacing between the vacuum tube's elements were fractions of an inch. Today, devices (transistors and diodes) in very large scale integrated circuits typically operate at three to five volts and have element and insulation dimensions of thousandths of an inch (microns). With the micron element spacing of integrated circuit devices, even relatively small ESD events can be catastrophic. The electronics industry has addressed the ESD problem in various ways. One way is to add ESD protective devices into an electronic system so that ail input and output lines are clamped below an ESD voltage that would damage the integrated circuits. Using an added ESD protective device, however, does not prevent integrated circuit ESD damage before the protective device is connected to the integrated circuit. Integrated circuit manufacturers have thus tried to incorporate some form of ESD protection into the integrated circuits themselves. Various forms of ESD protective devices such as zener diodes, capacitors and other controlled break down or surge filtering devices have been used to protect integrated circuit input/output (“I/O”) lines and typically may be connected between the I/O lines and both power supply rails (VDD and ground). These ESD protective devices, however, have a significant amount of parasitic capacitance (capacitance associated with the structure of the ESD device). This parasitic capacitance may degrade the high frequency signal performance of the I/O lines of the digital integrated circuit that are connected to the ESD protective devices. In addition, the ESD protective devices generally require series resistors, for example, of about 300 ohms in series with each signal path. This combination of significant parasitic capacitance and relatively high series resistance in each of the signal paths of the I/O lines may prevent or impair proper performance of high frequency signals, especially digital signals having very fast rising and falling transition levels, i.e., logic 0 to logic 1, and logic 1 to logic 0, respectively.
What is needed is a system, method and apparatus for electrostatic discharge protection which does not significantly degrade high frequency signal performance.